A rich array of habitats surround estuaries. The type of habitat is usually determined by the local geology and climate. Habitats associated with estuaries include salt marshes, mangrove forests, mud flats, tidal streams, rocky intertidal shores, reefs, and barrier beaches.
Examples of nearly every type of estuarine habitat exist along the coastline of the United States. In New England, salt-tolerant grasses fill salt marshes along the shores of tidal rivers. As one travels further south, the Atlantic Coast becomes much sandier, and barrier beaches enclose huge bays or sounds. In this region, estuarine habitats cover large areas along tidal rivers, and salt marshes reach far inland. Along the southern coast of Florida and lining the Gulf of Mexico are extensive mazes of mangrove forests, also called mangals.
From northwestern Florida to the Texas coast are long, narrow, sandy barrier islands and shallow estuaries lined with marshes. Along the Texas coast, barrier islands protect estuaries that have formed narrow lagoons with small openings to the Gulf of Mexico. In these areas, estuaries with very little freshwater input often become hypersaline or super salty.
Along the Pacific Coast of the United States, from northern California to Alaska, coastal rivers flow quickly out of the mountains and into very small estuaries. San Francisco Bay is one of the largest estuaries on the U.S. West Coast, and one of only a few that is similar in size to those found on the East Coast.
Levees are areas of higher ground that border the marsh creeks. Between the levees and tidal creeks are marsh flats, which contain pools and salt pannes. Salt pannes are shallow depressions that contain very high concentrations of salt. Pannes retain seawater for very short periods of time. When the seawater evaporates, the salts remain and accumulate over many tidal cycles. Glasswort, a plant tolerant to very high salt concentrations, is one of the only organisms able to survive in salt pannes. Pools are generally deeper than pannes, and retain water all year long. Salt-marsh snails and green crabs are some of the creatures found in pools scattered across the marsh.
Low-lying areas of the marsh are often covered with large, flat expanses of mud called mud flats. Composed of fine silts and clays, mud flats harbor burrowing creatures including clams, mussels, oysters, fiddler crabs, sand shrimp, and bloodworms.
Salt marshes are salty because they are flooded by seawater every day. They are marshy because their ground is composed of peat. Peat is made of decomposing plant matter that is often several feet thick. Peat is waterlogged, root-filled, and very spongy. Because salt marshes are waterlogged and contain lots of decomposing plant material, oxygen levels in the peat are extremely low—a condition called hypoxia. Hypoxia promotes the growth of bacteria which produce the rotten-egg smell that is attributed to marshes and mud flats.
Salt marshes are covered with salt-tolerant plants, or halophytes, like salt hay, black rush, and smooth cordgrass. However, these plants do not grow together in the same area. Marshes are divided into distinct zones, the high marsh and the low marsh. The difference in elevation between these two areas is usually only a few centimeters, but for the plants that inhabit each of these zones, a few centimeters makes a world of difference. The low marsh floods daily at high tide. The high marsh usually floods about twice a month during very high tides associated with new and full moons. The more often an area is flooded, the more saline it is. Plants living in salt marshes have different tolerances to salt. Those with higher tolerances are found in the low marsh, and those with lower tolerances to salt are found in the high marsh zones. Plants from one marsh zone are never found in the other.
Smooth cordgrass (Spartina alterniflora) dominates the low marsh all the way down to the estuary’s edge. It is tall, sturdy, broad-leaved, and one of the main components of peat. As one moves toward the high marsh, salt hay (Spartina patens), a very fine-leaved grass about 1-2 feet tall, and spike grass (Distichlis spicata) dominate the area. The highest parts of the marsh are characterized by black rush (Juncus gerardii), which grows in dense swaths.
Surrounding the high marsh are the upland habitats. Uplands are rarely, if ever, flooded with saltwater.
Mangrove forests or mangals grow at tropical and subtropical latitudes near the equator where the sea surface temperatures never fall below 16°C. Mangals line about two-thirds of the coastlines in tropical areas of the world.
There are about 80 species of mangrove trees, all of which grow in hypoxic (oxygen poor) soils where slow-moving waters allow fine sediments to accumulate. Many mangrove forests can be recognized by their dense tangle of prop roots that make the trees appear to be standing on stilts above the water. This tangle of roots helps to slow the movement of tidal waters, causing even more sediments to settle out of the water and build up the muddy bottom. Mangrove forests stabilize the coastline, reducing erosion from storm surges, currents, waves and tides.
Just like the high and low areas of salt marshes where specific types of grasses are found, mangals have distinct zones characterized by the species of mangrove tree that grows there. Where a species of mangrove tree exists depends on its tolerance for tidal flooding, soil salinity, and the availability of nutrients. Three dominant species of mangrove tree are found Florida mangals. The red mangrove (Rhizophora mangle) colonizes the seaward side of the mangal, so it receives the greatest amount of tidal flooding. Further inland and at a slightly higher elevation, black mangroves (Avicennia germinanas) grow. The zone in which black mangrove trees are found is only shallowly flooded during high tides. White mangrove (Laguncularia racemosa) and buttonwood trees (Conocarpus erectus), a non-mangrove species (Florida Department of Environmental Protection, 2000), face inland and dominate the highest parts of the mangal. The zone where white mangrove and buttonwood trees grow is almost never flooded by tidal waters.